This invention relates generally to head positioning in magnetic hard disk drives. More specifically, the present invention relates to a method for writing head position reference servo patterns on a magnetic data storage disk by employing a servowriter to record a coarse position reference burst pattern, and then by employing the disk drive servo to self-write a final fine position burst reference pattern based on the coarse position burst reference pattern.
Magnetic hard disk drives conventionally employ rotary voice coil head positioners for rapidly moving a data transducer head between concentric data tracks during track seeking operations and for maintaining the head over a selected data track during user data writing or reading operations. The data transducer head is maintained in very close proximity to a rotating magnetic data storage surface by flying on an air bearing at approximately one microinch, or less, above the surface. In this way, lineal data storage densities can be made very high. As lineal recording densities have begun to approach practical limits, another more recent trend for increasing storage capacity has been to make effective head magnetic widths narrower and narrower, in order to reduce track width and thereby increase the number of concentric data tracks that may be defined on a disk storage surface of standard manufacturing size, such as 3.5 inches in diameter.
Dual head structures have been adopted in order to recover discernable flux transition signals recorded on the disk. An inductive write element is used to write flux transitions onto the disk, while a separate read element of magneto-resistive or giant magneto-resistive material is used to read the flux transitions from the disk. It is known to provide a relatively wide write element, and a relatively narrow read element. In fact, narrower read elements are presently preferred because they permit a track to be read even though the head may not be precisely registered with, or maintained at, a track centerline by the head positioner. This relaxed tracking tolerance means that a less complex and expensive head position tracking system (servo) can be employed within the disk drive product. The use of separate write and read elements necessitates different tracking target positions for reading and writing, because of the presence of manufacturing tolerances. When a rotary head positioner is used, the write-to-read offset (the radial separation between the optimum tracking targets for writing and reading on a given track) clearly varies across the stroke of the rotary positioner as a function of the skew angle of the head support structure (air bearing slider body) relative to the recorded track. This offset variation at any particular position is known as the xe2x80x9cwrite-to-read offsetxe2x80x9d. One drawback of narrow read elements is that such elements make it difficult to obtain a good position error signal (xe2x80x9cPESxe2x80x9d) at all positions across the radial width of a data track when used with a traditional burst pattern, for example the pattern described in commonly assigned U.S. Pat. No. 5,170,299 to R. Moon, entitled: xe2x80x9cEdge Servo for Disk Drive Head Positionerxe2x80x9d, the disclosure thereof being incorporated herein by reference. This drawback requires provision and use of a special servo burst pattern that provides a usable PES at virtually all radial head positions, relative to actual center, for each track across the actuator stroke.
The write and read elements of dual head structures can have certain positional offsets, particularly when a rotary voice coil actuator is used to position dual-element heads. With a rotary actuator the positional offsets between the write and read elements vary over the rotational stroke of the actuator and head transducer relative to the disk surface.
Because the data tracks are placed very close together (high track density of 1,000 to 20,000 tracks, or greater, per disk radial inch) a head position servo loop is typically provided as part of the disk drive electronics in order to control the head positioner. In order to provide head position information to the servo loop, each magnetic storage surface typically carries recorded servo information. This information is most frequently xe2x80x9cembeddedxe2x80x9d within each data track as a circumferential series of narrow servo sectors between user data sectors or segments, sometimes referred to as xe2x80x9cwedgesxe2x80x9d or xe2x80x9cspokesxe2x80x9d. The servo information typically comprises certain phase-coherent digital information used during track seeking and coarse positioning operations, and fine position information typically in the form of burst patterns used for head tracking during reading and writing operations from and to a particular track. Once written during a servowriting step within the disk drive manufacturing process, servo sectors are thereafter protected by disk drive controller logic from overwriting as being denominated xe2x80x9cread-onlyxe2x80x9d areas of the disk""s storage surface. As the head passes over a servo sector location, coarse and fine position information is sampled by the head, and this sampled information is provided to, and used by, the disk drive""s servo control loop for closed loop control of the head positioning apparatus during track seeking and track following operations.
In order to provide precisely written servo information, very precise servowriting instruments, typically employing laser-based interferometer technology, are employed within xe2x80x9cclean roomsxe2x80x9d of the drive manufacturing facility wherein atmospheric particulate contamination is closely controlled. Clean rooms are required because the disk drive is typically servo-written with its interior exposed to the ambient environment. The laser-interferometer servowriter precisely measures actuator position of the disk drive. Based upon this precisely measured position, a drive head positioner, such as a rotary voice coil actuator, is moved under servowriter control from track to track while each data head in turn writes the servo information to an associated data storage surface. Once the servo pattern is written, it can be tested by a read back procedure while the drive remains at the servowriter station in order to verify that the servo patterns have been effectively and accurately written. It is known to write servo bursts with three passes per track under servowriter control. However, with a high number of tracks on each disk surface, the servo burst writing process can become very time consuming and therefore very expensive.
Representative examples of disk drive servowriters and servowriting techniques are provided in U.S. Pat. No. 5,748,398 to Seo, entitled: xe2x80x9cMethod for Writing Servo Signals onto a Magnetic Disk and Magnetic Disk Drive Equipped with Magnetic Disk(s) Having Servo Pattern Recorded by the Methodxe2x80x9d; U.S. Pat. No. 5,726,879 to Sato, entitled: xe2x80x9cControl Apparatus, a Stage Apparatus and a Hard Disk Servowriter Apparatus Including a Robust Stabilizing Compensatorxe2x80x9d; U.S. Pat. No. 5,627,698 to Malek, entitled: xe2x80x9cServo Information Recording Method and Apparatus for an Enclosed Data Storage Systemxe2x80x9d; U.S. Pat. No. 5,339,204 to James et al., entitled: xe2x80x9cSystem and Method for Servowriting a Magnetic Disk Drivexe2x80x9d, the disclosures thereof being incorporated herein by reference. One characteristic which is common to servowriters is that they are very complex and expensive items of capital equipment within the disk drive manufacturing process. Servowriter and related clean room costs must be amortized as an incremental cost burden of each disk drive being produced and servowritten.
It has been proposed to write a servo pattern on a surface of a reference disk with a servowriter. Following installation of the reference disk onto a disk drive spindle, the reference servo patterns are used to write embedded servo patterns onto other storage surfaces within the disk drive. Such approach is described by H. Ono, in an article entitled: xe2x80x9cArchitecture and Performance of the ESPER-2 Hard-Disk Drive Servowriterxe2x80x9d, IBM J. Res. Develop. Vol. 37, No. 1, January 1993, pp. 3-11. One drawback of the approach described by Ono is that a center of revolution of the reference disk on the servowriter may not correspond to a center of revolution of the reference disk in the disk drive, and that non-repeatable runout errors in radial and tangential dimensions differ between the different disks in the disk drive. (Tangential tracking errors interfere with servo information phase coherency and impose limitations upon servo clocking speed for the phase coherent digital servo information fields).
Since a data storage disk surface may contain media defects or anomalies, it has been proposed to write multiple servo pattern sets and then select an error free set, while overwriting (erasing) the other sets. This approach is described in commonly assigned U.S. Pat. No. 5,553,086 to Sompel, et al., entitled: xe2x80x9cMultiple Servo Sector Sets Write With Self-Verification for Disk Drivexe2x80x9d, the disclosure thereof being incorporated herein by reference.
As already mentioned the embedded servo information typically comprises certain digital data followed by certain fine position bursts recorded at a fixed frequency used for following a particular data track. The digital data desirably remains phase-coherent from track to track so that it can be read during track seeking operations, and also read while the servo system is track-following between two tracks. The fine position bursts are circumferentially sequential and radially offset, so that as the head passes over fractional portions of burst sets, fractional burst amplitude samples are read. These amplitude samples are compared and used by the servo system to generate a PES to control head position during track following operations when reading and writing is carried out. Because of the write-to-read offset or offset between the read and write elements of a dual head structure at a particular track location, micro-jogging operations may be employed for proper head positioning. A disk drive having a head transducer comprising a relatively wide inductive write element and a relatively narrow magneto-resistive read element, and wherein the head transducer is positioned by micro-jogging a rotary voice coil actuator, is illustrated and described in commonly assigned U.S. Pat. No. 5,587,850 to Ton-that, entitled: xe2x80x9cData Track Pattern Including Embedded Servo Sectors for Magneto-Resistive Read/Inductive Write Head Structure for a Disk Drivexe2x80x9d, the disclosure thereof being incorporated herein by reference.
Following servowriting and while remaining within the clean room environment, the disk drive head-disk- assembly (xe2x80x9cHDAxe2x80x9d) is sealed to prevent external particulate contamination. After the HDA has been sealed and moved out of the clean room, an electronics circuit board is connected to the HDA to complete the physical assembly of the drive. At this stage, the fully assembled disk drive is sent to a burn-in rack or self-scan station where it is typically operated continuously over a period of time, and also typically over a range of temperatures, to assure reliability. Also, during self-scan, the drive conducts certain self-scan operations and discovers and develops certain facts and characteristics about itself, such as the reliability and characteristics of the heads and storage disks, and the locations of any media defects. These data are then typically recorded on reserved tracks of the disk drive and may be used later during normal disk drive operations in order to maintain and control drive performance. Also, during self-scan, certain configuration and operational firmware and software may be transferred to reserved tracks of the disk(s) for later use by the embedded disk drive controller during normal drive data storage and retrieval operations.
As the number of data storage tracks per disk surface (track density) increases, servowriter accuracy and writing time proportionally increases. While it is theoretically possible to provide an unlimited number of expensive servowriters within a clean room drive manufacturing environment, in practice a limited number of servowriters only are available, and servowriting time can become a manufacturing bottleneck, particularly as newer disk drive designs have included storage surfaces having thousands of data tracks. Also, it would be desirable to utilize servowriters of a given writing accuracy over a number of product cycles, each cycle typically manifesting increased track density.
Burn-in or self-scan racks are far less expensive than servowriters, and adding self-scan rack capability to the manufacturing process raises the burdened costs of the drive far less than adding servowriting capacity.
One proposal to reduce the cost of servowriting disk drives has called for moving the servowriter out of the expensive clean room. This approach calls for localized ambient air purification and scrubbing as by injecting clean air into a clock head port of the disk drive in a positive pressure arrangement such that the injected air exits the drive at the servowriter push-pin port. While this is a cheaper approach than the clean room, it still requires an expensive servowriter apparatus, and the possibility of particulate contamination entering the disk drive interior is greater than from using the clean room environment.
Several self-servo-writing methods and algorithms have been proposed in an attempt to avoid the cost and inconvenience of the servowriter entirely. One such approach is described in commonly assigned U.S. Pat. No. 5,668,679 to Swearingen et al, entitled: xe2x80x9cSystem for Self-Servowriting a Disk Drivexe2x80x9d, the disclosure thereof being incorporated herein by reference. Other examples are found in U.S. Pat. No. 5,448,429 to Cribbs et al. entitled: xe2x80x9cSelf-Servowriting Disk Drive and Methodxe2x80x9d; U.S. Pat. No. 5,541,784 to Cribbs et al. entitled: xe2x80x9cBootstrap Method for Writing Servo Tracks on a Disk Drivexe2x80x9d; U.S. Pat. No. 5,757,574 to Chainer et al., entitled: xe2x80x9cMethods and Systems for Self-Servowriting Including Maintaining a Reference Level Within a Usable Dynamic Rangexe2x80x9d; U.S. Pat. No. 5,793,554 to Chainer et al., entitled: xe2x80x9cSelf-Servowriting System with Dynamic Error Propagation Reductionxe2x80x9d; U.S. Pat. No. 5,570,247 to Brown et al., entitled: xe2x80x9cSelf Servowriting Filexe2x80x9d; and U.S. Pat. No. 4,414,589 to Oliver et al., entitled: xe2x80x9cEmbedded Servo Track Following System and Method for Writing Servo Tracksxe2x80x9d, the disclosures thereof being incorporated herein by reference.
While complete self-servowriting is a highly desirable goal, it is very difficult to realize in practice, given manufacturing tolerances in head widths, gains, alignments, storage media characteristics and quality, etc. Simply put, self-servowriting procedures so far have proven problematic in disk drive mass production in providing digital servo information that remains phase coherent from track to track across the data storage surface, and in establishing sufficiently accurate positioning for servo bursts needed to provide linear PES values for current data track densities, which are quickly approaching 20,000 tracks per inch. Such high track densities not only require more precisely written servo reference patterns, but also high bandwidth servo control loops. High bandwidth servo loops may be implemented by use of dual-stage actuators, for example. One dual-stage actuator employing a piezoelectric device within a magnetic head arm is described in U.S. Pat. No. 5,189,578 to Mori et al., entitled: xe2x80x9cDisk System with Sub-Actuators for Fine Head Displacementxe2x80x9d, the disclosure thereof being incorporated herein by reference.
Therefore, a hitherto unsolved need has remained for a servowriting procedure which minimizes actual servowriter time while enabling the disk drive to self-servo-write embedded servo burst patterns supporting very high data track densities during drive burn in without need for writing coherent patterns outside of the servowriter environment.
One object of the present invention is to reduce the amount of time that a disk drive spends at a servowriter station during disk drive manufacturing in a manner overcoming limitations and drawbacks of the prior art.
Another object of the present invention is to improve the quality of product servo burst patterns by removing certain unwanted pattern artifacts, such as those attributable to disk vibration, during a self-servo-writing process following reference pattern writing within a servowriter environment.
A further object of the present invention is to provide a method for self-servo-writing of magnetic hard disk drives based on reference servo patterns written with the aid of a servowriter in a manner overcoming limitations and drawbacks of the prior art.
Yet another object of the present invention is to employ a servowriter within a disk drive manufacturing process to record phase-coherent digital servo information and a reference burst pattern, and then use the completed disk drive to self-write more detailed and comprehensive servo burst patterns derived from the reference burst pattern during extended self-scan operations.
As a related object of the present invention, a servowriter writes an initial untrimmed three-burst-per-two-track burst pattern, and the disk drive writes a product burst pattern at one-third track pitch intervals by following the initial three-burst-per-two track burst pattern without determining write element width or read element width, so long as write-to-read offset is determined.
One further object of the present invention is to extend the useful service life of servowriters over a number of generations of disk drive products wherein each generation has a track density increased from a prior generation, without need for upgrading the servowriter to match the highest or latest track density layout or design.
Yet one more object of the present invention is to produce a servo format with sufficient PES linearity to enable use of very narrow magnetoresistive read elements relative to writer width (e.g., less than 40 percent of track pitch) without increasing servowriter time.
Yet another object of the present invention is to transfer a significant portion of disk drive servowriting activity from an expensive servowriter environment to the disk drive within a less expensive drive self-scan environment, resulting in disk drives made at less expense and with greater reliability.
Still another object of the present invention is to servowrite a family of hard disk drives having heads characterized by wide reader/writer tolerances in a manner which optimizes manufacturing utilization of clean room servowriters by having each disk of the family self-write at least significant portions of a final embedded servo burst pattern on each storage surface in accordance with particular head characteristics within each drive.
In accordance with principles of the present invention, a method for servowriting a magnetic hard disk of a head-disk assembly comprises the following steps:
servo-writing a reference servo burst pattern using a servowriter coupled to the head-disk assembly at a servo-writing station within the disk drive manufacturing operation,
completing assembly of the disk drive by attaching and connecting an electronics board to the head-disk assembly within the manufacturing operation,
transferring the completed disk drive to a burn-in rack,
transferring certain self-servo-write control software to the disk drive, and
operating the disk drive at the burn-in rack to self-write at least a portion of a final servo burst pattern by using the reference burst pattern written to the head-disk assembly by the servowriter. In this regard, the disk drive may self-write intermediate servo bursts patterns which are used to self-write the final servo burst pattern in order to take into account write-to-read offset from a rotary positioner of the disk drive.
In a related aspect of the present invention, a magnetic hard disk drive has at least one data storage disk rotated by a spindle motor and at least one head transducer comprising an inductive write element and a magneto-resistive read element and positioned at radial track locations defined on a storage surface of the disk by a head positioner. Upon final assembly the disk drive includes a write/read channel connected to the head transducer, a spindle driver for driving the spindle motor, a positioner driver for driving the head positioner, an interface for connecting the drive to an external computing environment and including a cache buffer, and a drive controller for controlling at least the head positioner to position the head transducer at selected data tracks. In this example of the invention the disk drive has reference servo patterns recorded onto at least a part of the storage surface by a servo writer as a part of a manufacturing process, the reference servo patterns being incomplete with respect to a final product pattern. After final assembly the disk drive is loaded with and executes self-servo-writing software for self-writing embedded servo final product patterns across the storage surface based upon the reference servo patterns. The reference servo patterns may be part of the final product pattern, or they may be discarded and overwritten. In addition, intermediate servo burst patterns may be written for some tracks in order to compensate for write-to-read offsets and/or differences between relatively wide write element magnetic width (e.g. 66-120% of track width) and relatively narrow read element magnetic width (e.g. 35-75% of track width). Calibration processes carried out within the self-servo-writing process determine the need for, and location of, any intermediate servo burst patterns. Phase coherent servo fields including track number information may also be included within the reference servo patterns and the final product servo patterns.
In one more example including principles of the present invention, a disk drive comprises a magnetic data storage disk having a storage surface defining an embedded servo pattern. The drive also includes a head transducer comprising a magnetic write element having a magnetic writing width in a range between 66% and 120% of nominal track width, and a magneto-resistive read element having a magnetic reading width in a range between 35% and 75% of the nominal track width, and a head positioner for positioning the head transducer relative to concentric data storage tracks further defined on the storage surface. In this example the embedded servo pattern includes for each pair of adjacent data tracks a six servo burst pattern of circumferentially sequential, radially offset untrimmed bursts A, B, C, D, E and F. A four servo burst pattern per track employing trimmed bursts may also be employed with slightly less tolerance for head width variances; however, one drawback of trimming and writing bursts in the same pass is that there is less randomness along each trimmed burst edge and therefore higher written-in repeatable runout (xe2x80x9cRROxe2x80x9d).
These and other objects, advantages, aspects, and features of the present invention will be more fully appreciated and understood upon consideration of the following detailed description of preferred embodiments presented in conjunction with the accompanying drawings.